The conversion capability of catalytic converters of this kind is dependent upon deterioration and can be damaged because of a malfunction of the engine and because of erroneous use by the driver of a motor vehicle. Statutory requirements provide for an on-board diagnosis of the motor vehicle components which are relevant to the emission of toxic substances, such as catalytic converters. U.S. Pat. No. 3,969,932 discloses a diagnosis of a catalytic converter which is based on the evaluation of the phase shift between the signals of an oxygen probe mounted forward of the catalytic converter and an oxygen probe mounted rearward thereof.
The known diagnosis utilizes the relationship of the oxygen storage capability of the catalytic converter with its conversion capability. For this purpose, an operation of the engine first takes place with an oxygen deficiency caused by a fuel enrichment of the mixture of fuel and inducted air which is to be combusted in the engine. Oxygen, which is possibly stored in the catalytic converter, reacts with the excessive fuel so that, after a certain time, a catalytic converter is present which has been definitively emptied of oxygen. Correspondingly, the exhaust-gas probes, which are mounted forward and rearward of the catalytic converter, signalize an oxygen deficiency. Thereafter, the engine is operated with an oxygen enriched mixture. The oxygen excess resulting therefrom in the exhaust gas is first stored in the catalytic converter so that, at first, an oxygen deficiency continues to be present rearward of the catalytic converter. This changes only after the oxygen storage capability of the catalytic converter is exhausted. The probe, which is mounted rearward of the catalytic converter, therefore reacts with a delay to the change of the oxygen content of the exhaust gas. The extent of the delay, which becomes manifest as a phase shift between the signals of the two probes, reflects the storage capacity and therefore the converting capability of the catalytic converter.
It has been shown that phase shifts of typically 200 ms occur which are burdened with a measuring inaccuracy of approximately 30 ms. The measuring uncertainty is caused, for example, by temperature influences. The phase shift to be expected increases with increasing size of the catalytic converter. For this reason, the measurement uncertainty remains essentially constant and the known diagnosis is that much better the larger the catalytic converter to be evaluated. For smaller catalytic converters, for example, precatalytic converters, for vehicles having forward and main catalytic converters, the known diagnosis is therefore only suitable to a certain extent.